We use a motorcycle filler tube and cap on most of the non-original ones we do. It turns a quarter turn counter clockwise and pops up to unthread from there just like a new car. If one chooses to paint body color it is nearly invisible. Can you spot the cap about 2" under the body molding line on the blue 34?

http://s32.photobucket.com/user/coilover/media/015.jpg.html
http://s32.photobucket.com/user/coilover/media/016.jpg.html
http://s32.photobucket.com/user/coilover/media/two%2034s%20outside%20001.jpg.html

Evan